This invention relates to a method and a system for acquiring ultrasound tissue characterization values for example, the ultrasound propagation velocity, the scattered ultrasound waveform, etc., for tissues of a living subject, in connection with an ultrasound image, so that the structure of the tissue of the living subject may be determined.
The ultrasound propagation velocity and the scattering of the ultrasound waveform, for example, in living tissue, are influenced by the intervening tissue or organs situated along the propagation path. This fact is of clinical value because the development of a disease such as a tumor in the living tissue can be ascertained by measuring the ultrasound propagation velocity and the scattered ultrasound waveform. Attempts have been made to evaluate the tissue characterization of a region of interest (ROI) in living tissue by measuring the ultrasound propagation velocity and the scattered ultrasound waveform in the living tissue. A method such as that shown in FIG. 1, in which a linear electronic scan type ultrasound diagnostic apparatus is used, has been proposed as the aforementioned evaluating ultrasound measuring method. In FIG. 1, an ultrasound pulse is transmitted at an angle of .theta. from one end an of ultrasound transmitting/receiving surface 2 adjacent to, for example, the tissue surface (not shown), toward the body of the patient by means of a ultrasound type linear electronic scan probe 1. Here, the electronic scan type ultrasound apparatus uses a probe which comprises a linear array of ultrasound oscillating elements. Some of the adjacent oscillating elements in the probe receive drive pulses, each having a predetermined delay time which is determined in accordance with the direction of the ultrasonic wave or beam transmission and the position of the oscillating element, and are transmitted as an ultrasound oscillation, wave or beam. The transmitted ultrasonic beam travels straight along transmission path 3, for example, in the living tissue, and is echoed back or reflected at a point P. The reflected ultrasonic wave or beam is received at the other end b of probe 1 along receiving path 4, instead of being directed toward the aforementioned oscillating elements at end a.
This is known as a cross beam method, in which the ultrasonic wave crosses at the point of intersection of transmitting path 3 and receiving path 4. The scattered waveform of the ultrasound beam thus obtained contains various data relating to the tissue structure of the examined part of the living body.
Since, in the arrangement shown in FIG. 1, the distance y between the ends a and b of probe 1 is already known, then if the propagation time t of ultrasonic wave travelling along transmitting and receiving paths 3 and 4 is measured, the propagation velocity C of the ultrasonic wave, travelling through the living tissue can be given by: EQU C=y/(t.multidot.sin.theta.) (1)
If the average velocity C.sub.0 of the ultrasonic propagating through the living tissue is 1530 [m/s], and, the distance between the adjacent oscillating elements is represented by d, the delay time .tau..sub.0 between the adjacent oscillating elements can be given by EQU .tau..sub.0 =(d/C.sub.0).multidot.sin.theta..sub.0 ( 2)
in order to transmit the ultrasonic wave at an angle of .theta..sub.0. In this way, the aforementioned delay time is set to permit the respective oscillating elements to be driven accordingly.
Where the propagation velocity of the ultrasonic wave travelling through the living tissue is represented by C, then the ultrasonic wave travels in a direction given by the angle .theta.. Since, in general, C is different from C.sub.0, the direction given by the angle .theta..sub.0 at which the ultrasonic wave propagates is represented by the following equation using the Snell's laws. EQU sin.theta./C=sin.theta..sub.0 /C.sub.0 ( 3)
Equation (3) being substituted into Equation (1), the ultrasound propagation velocity C becomes: ##EQU1## Substituting Equation (2) into Equation (4) yields ##EQU2## With y, d, and .tau..sub.0 already being known, Equation (4') can be evaluated using ultrasonic wave propagation time t, which is determined by means of the cross beam method. In this way, the ultrasound propagation velocity C is obtained.
FIGS. 2A through 2D are timing charts for illustrating the method for evaluating the ultrasonic wave propagation time t. In FIG. 2A, the ultrasound pulse is transmitted at a time somewhat behind the time t.sub.0 at which the rate pulse rises. Here, t.sub.1 represents the time corresponding to the peak amplitude value of the ultrasound pulse. Where a reflector is located at the intersection between the ultrasound transmitting direction and the receiving direction, the ultrasound propagation time t is ascertained at time (t.sub.2 -t.sub.1), when the receiving waveform is obtained which has a peak amplitude value at time t.sub.2.
In fact, it is rare that a point reflector is located within the living tissue and, where the ROI, for example, in the living tissue, is relatively uniform so that the reflected ultrasonic wave emerges as a relatively uniform received waveform. As is shown in FIG. 3, a ROI with a certain width is located at the points P1, P2, with the intersection P as a center, because the transmitted ultrasound beam has a finite width. That is, the ultrasonic wave first reaches the probe by reflection at point P1, and the last portion of the ultrasonic wave reaches the probe by reflection at point P2. For this reason, the received waveform has a time width corresponding to the range from point P1 to point P2. In this case, the received waveform becomes a broadened scattered waveform, as is shown in FIG. 2B, and thus, due to the living tissue not being completely uniform, the ultrasonic wave scattered at various locations in the living tissue is received as a synthesized signal component. In this way, a random received waveform appears. Since, however, it is not possible to detect the peak amplitude value from the random received waveform, the position of the probe is somewhat displaced so that the location of the aforementioned intersection within the living tissue may be moved. The received waveforms obtained at these times, if additively averaged for example, appear to vary smoothly in a wavy fashion, as is shown in FIG. 2C. If, in addition to the aforementioned method, curve fitting is also employed, using a least squares method, through the use of a unimodal function with a single peak, it is possible to obtain a very smooth curve, such as is shown in FIG. 2D.
A method for measuring the ultrasound propagation velocity has also been proposed which employs a symmetric measuring system to which the cross beam method is applied. This is known as a 4-beam method, as is disclosed in more detail in FIG. 4. As is shown in FIG. 4, in order to measure the ultrasound propagation velocity at reflection measuring points P.sub.11 and P.sub.12 (the upper boundary), and at a reflection measuring point P.sub.00 (the lower boundary), four ultrasound beam transmitting/receiving signal routes are established: (R1) a.fwdarw.P.sub.00 .fwdarw.b, (R2) a.fwdarw.P.sub.11 .fwdarw.c, (R3) b.fwdarw.P.sub.00 .fwdarw.a; and (R4) b.fwdarw.P.sub.12 .fwdarw.d. This provides a symmetric measuring route, to allow the same angle .theta. to the ultrasound transmitting/receiving directions.
According to this method, symmetric measurements are made twice per route at the forward path and at the backward path. In the evaluation of the ultrasound propagation velocity, the average value can be obtained, to reduce any operational error. The ultrasound propagation velocity measuring function employing the cross beam method is incorporated into the ultrasonic apparatus. The values obtained, together with the ultrasound image such as a B mode, is displayed in real time on a display unit. This display is shown in FIG. 5. In FIG. 5, marker 10 represents a setting route for measuring the ultrasound propagation velocity at the ROI of the living body and marker 10 shown on B mode image 11 which is measured in real time. A mode images 13a, 13b, obtained by measuring the ultrasound propagation velocity using the aforementioned cross beam method, respective ultrasound propagation velocity values 14, and time variation diagram 15 of the average ultrasound propagation velocity value, are displayed on the other area of display 12. Marker 10 shows routes R1 through R4 as set forth above in connection with FIG. 4. Ultrasound propagation velocity value 14 is V1 for route R1, V2 for route R2, V3 for route R3 and V4 for route R4, V indicates the average value of these four values V1 to V4. A mode images 13a and 13b are the ultrasound echoes passing through route R1 or R3 and route R2 or R4, respectively. Gates 16a and 16b showing the A mode are set to extract only the ultrasonic wave echoed back from the ROI and to eliminate those ultrasound echoes back from those area adjacent to the ROI. These gates 16a and 16b with predetermined width are set based on the propagation time obtained from the ultrasound propagation velocity and distance to the ROI in the living body. In the image display, B mode image 11 is sequentially measured in real time, and updated, while the ultrasound propagation velocity is measured in the aforementioned cross beam method on the aforementioned four routes.
FIG. 6 shows one form of display at a "freeze" time as substantially similar to a real time display. At the "freeze" time, scattered waveform width 20 and average A mode images 21a, 21b for the respective routes are also displayed on display 12.
In the conventional system, of the living tissue characterization values, the average value of the ultrasound propagation velocity is repetitively measured and displayed. On the other hand, the scattered waveform is measured and displayed at the "freeze" time only, as shown in FIG. 6.
However, for example, the tissue structure of the ROI in the liver is not uniform during the development of the disease and the scattered waveform is consequently varied by the disease as various kinds of scattering objects of assume various sizes, and as the abnormal tissue regions microscopically invade the normal tissue. For this reason, the state of the disease cannot be exactly determined, often causing difficulty in determining the presence of the disease. It is, therefore, required that the scattered waveform be measured as the ultrasound propagation velocity.